Metal oxides having semiconductor characteristics, and titanium oxide in particular, are endowed with excellent photocatalytic activity by being irradiated under ultraviolet illumination, and chemical substances in the gas or liquid phase (also the solid phase in certain cases) is decomposed oxidatively on the surface thereof. The titanium oxide having strong oxidizing power decomposes chemical substances by the photocatalytic reaction and exhibits deodorizing, antifungal, and bactericidal characteristics.
Vapor deposition and sol-gel techniques are widely used as conventional techniques for supporting titanium dioxide on the surface of titanium or a titanium alloy. The sol-gel technique is a simple method performed by dip coating and spin coating, and has the ability to provide support on arbitrary morphologies of base materials, but is problematic in being an inferior method for mass-production. Particular weaknesses are that film uniformity is poor and support can be provided only on plate base materials with spin coating. On the other hand, vapor deposition provides excellent density and uniformity in titanium oxide, but is disadvantageous in that the cost is high due to the use of special devices, and the possibility of spalling is high due to considerable film thickness.
A method in which a titanium dioxide thin film having photocatalytic activity is formed by anodic oxidation has been proposed as an alternative to the previous techniques. In the reported examples of the manufacture of photocatalytic titanium dioxide thin films using anodic oxidation, it is assumed that, as a photocatalyst, anatase-structured titanium dioxide has higher activity than does the rutile-structured titanium dioxide known as a high-temperature crystal, and the object is to form an entirely anatase-structured titanium dioxide film.
Patent Document 1 discloses as such a conventional technique a photocatalyst and a photocatalyst manufacturing method characterized in that the crystal structure includes rutile-structured titanium dioxide, but this is a technique for fabricating a photocatalyst composed of titanium oxide powder. The rutile-structured titanium dioxide powder in these techniques is obtained by annealing the anatase-structured titanium dioxide powder in hydrogen atmosphere, resultantly it is composed of a mixture with anatase-structured titanium dioxide in which oxygen defects are introduced; is obtained as rutile-structured titanium dioxide powder; or is obtained by introducing oxygen defects or substituting the third element in titanium oxide.
Among these, there are no documents reporting the super-hydrophilicity of titanium dioxide thin film by anodic oxidation, and the formation of a rutile-structured titanium dioxide thin film having photocatalytic activity by anodic oxidation is not reported.